This is the door to Cloudland, where the giant with the Magic Chest is located. In order to open the door, you will need a key from the Gnome in C2. You only get the key if you don't guess his name. When you do guess his name, you get beans to grow the bean stalk— and you won't need the key. Once in the Cloudland, you can come down though the caves rather than the bean stalk and still exit though this door—it is not locked on the other side. For points, use the bean stalk to get to the cloadland. Exit using the caves as it is a bit easier than climbing down the bean stalk.